Certain polymers, copolymers and blends demonstrate piezoelectric behavior due to dipolar ferroelectricity residing in specific crystal phases. The most common of the semicrystalline ferroelectric polymers are poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene, P(VDF-TrFE). As with any piezoelectrically active material, if electrodes are deposited on both of the major surfaces of a sample of piezoelectric film, the element functions as an electromechanical transducer and thus can be used as a sensor or actuator. If the film, operating as a sensor, is subjected to stress either in the direction corresponding to the film thickness, or in either of the transverse directions, an electric potential proportional to the applied stress is developed across the thickness of the film. The electrodes enable connection to external electronic circuitry, making it possible to process the information provided by the sensor.
Sensors having piezoelectric film transducers for passive detection of body sounds are known. See, e.g., U.S. Pat. Nos. 5,365,937 and 5,595,188 and published applications WO/92-08407 and WO/90-08506. However, known sensors may be characterized by undesirable performance constraints, including low signal to noise ratio, cross-talk, and signal contamination from power line harmonics or ambient room noise. These constraints may in part be attributable to transducer non-conformity to human body surfaces.
An improved thin-film sensor for the efficient passive detection of heart and blood-flow sounds is needed.